Dark force could keep Milky Way's neighbours away

Dark energy is thought to be ripping apart the fabric of space-time on cosmological scales, but it now seems it is also active on the scale of a single galaxy. If so, it could explain why the Milky Way has fewer dwarf galaxies orbiting it than expected.

Astronomers came up with dark energy in the late 1990s as a way to explain the discovery that the expansion of the universe is accelerating. But it has so far only been studied on scales spanning a significant fraction of the universe.

“Most people think that on shorter distance scales dark energy doesn’t do anything, or it’s completely undetectable,” says Stephen Hsu of Michigan State University in East Lansing. At short distances, the other forces – including gravity – are thought to be strong enough to counter dark energy’s repulsive force.

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That’s certainly true of atoms, molecules and even solar systems and the interior of galaxies. But Hsu and his colleagues wondered how far from the centre of a galaxy you had to go before dark energy took over.

“We were surprised when we ran the numbers,” says Hsu. “There could actually be an appreciable effect. Nobody had pointed this out before.”

Critical radius

Their calculations show that for every galaxy, there is a critical radius from the galactic centre where the gravitational influence of the galaxy’s mass is balanced by the repulsive force of dark energy.

For massive galaxies like our own Milky Way, which is about 100,000 light years across, this critical radius is about 1.6 million light years. That means nothing inside large galaxies would be affected by dark energy.

But for smaller dwarf galaxies, which are about four orders of magnitude less massive than the Milky Way and can be as small as a few hundred light years across, the critical radius is about 75,000 light years.

That gives astronomers an opportunity to test the idea&colon; at these distances, gas clouds that are bound to the dwarf galaxies should be orbiting at slower speeds than they would be without the influence of dark energy, says Hsu. “I hope the race will be on for astronomers to go and find systems where they can measure an effect,” he says.

James Schombert at the University of Oregon in Eugene calls it a “clever idea”, but suspects that most current observations wouldn’t tally with these results. However, “it’s a testable idea, and thus has merit”, he says. Testing it would require very sensitive telescopes, such as the upcoming Large Synoptic Survey Telescope and the Square Kilometre Array.

Galaxy outskirts

If such an effect were found, studying the way gas clouds move at the outskirts of dwarf galaxies could help distinguish between different models of dark energy. For instance, Hsu and colleagues assumed that dark energy is Einstein’s cosmological constant as each unit volume of space-time contains the same amount of dark energy. But if dark energy is not constant but instead interacts with gravity, then the critical radius would be shorter and its effects would show up in dwarf galaxies.

The galactic effects of dark energy could also solve a longstanding mystery called the missing satellite problem. Astronomers see far fewer dwarf galaxies orbiting the Milky Way than simulations of galaxy formation predict should be there. But according to Hsu’s calculations, dark energy should prevent anything from orbiting the Milky Way beyond its critical radius – explaining why no such satellites have been found.

“It’s possible that the missing satellite problem is just a manifestation of the fact that there is actually a repulsive force involved,” says Hsu.